1. Field of the Invention
This invention lies in the field of seismic geophysical prospecting, and is directed particularly to improvements in the generation of seismic waves and in their processing.
It concerns a seismic wave generator which comprises a vertical array of separate sources at known spacings which are initiated at an arbitrary selected known time sequence, different from that normally provided in a constant velocity type distributed charge.
Still more particularly, it concerns the reception of the series of timed seismic waves after travel throughout the earth, to form a first electrical signal, and correlating this signal with another signal which represents a series of pulses at the known time separations derived from the time sequence of initiations, as modified by the travel times between vertical source positions, of seismic waves in the surrounding medium.
2. Description of the Prior Art
There is considerable prior art both in publications and patents, of the use of long vertical arrays of spaced charges for use in boreholes. However, all of these without exception provide equally spaced charges, in a region of constant known rock velocity, and the equal time intervals chosen for successive detonations is equal to the travel time of seismic waves in the rocks over a distance equal to the spacing between successive charges in the array.
Furthermore, all of these so-called constant velocity, convel, and broomstick charges are detonated from the top downward, in order to concentrate the successive pulses into a single additive pulse which is directed downwardly.
The main basis for this use of separately spaced charges detonated in sequence with the travel time of the seismic wave in the rocks is based upon the principle that the downgoing waves from each of the charges reinforce each other, to provide a seismic pulse, or wave, which is the sum of the separate pulses from each of the charges. The ongoing waves comprise a series of spaced pulses, equal in number to the pulses generated by individual charges, which do not add in any way, but do provide down-going reflections from each of the separate charges which are reflected from reflectors above the array. These reflections complicate the seismic record.